Epitaxial substrate for electronic device and method of producing the same

ABSTRACT

An object of the present invention is to provide an epitaxial substrate for an electronic device, in which a lateral direction of the substrate is defined as a main current conducting direction and a warp configuration of the epitaxial substrate is adequately controlled, as well as a method of producing the epitaxial substrate. Specifically, the epitaxial substrate for an electron device, including: a Si single crystal substrate; and a Group III nitride laminated body formed by epitaxially growing plural Group III nitride layers on the Si single crystal substrate, wherein a lateral direction of the epitaxial substrate is defined as a main current conducting direction, is characterized in that the Si single crystal substrate is a p-type substrate having a specific resistance value of not larger than 0.01 Ω·cm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an epitaxial substrate for anelectronic device and a method of producing the same and, in particular,to an epitaxial substrate for a HEMT and a method producing the same.

2. Description of the Related Art

In recent years, HEMT (High electron mobility transistor) has beenwidely used as high-speed FET (Field effect transistor) as speedrequired of an IC device increases. Such a FET-type transistor asdescribed above is generally formed, as schematically illustrated inFIG. 1, for example, by laminating a channel layer 22 and an electronsupply layer 23 on an insulating substrate 21 and then providing asurface of the electron supply layer 23 with a source electrode 24, adrain electrode 25 and a gate electrode 26. When this transistor deviceis operated, electrons are moved through the source electrode 24, theelectron supply layer 23, the channel layer 22, the electron supplylayer 23 and the drain electrode 25 in this order, thereby defining alateral direction of the device as a main current conducting direction.This movement of electrons in the lateral direction, i.e. the maincurrent conducting direction, is controlled by voltage applied on thegate electrode 26. In a HEMT, electrons generated at a joint interfacebetween the electron supply layer 23 and the channel layer 22 of whichband gaps are different from each other can move significantly fast, ascompared with electrons in a conventional semiconductor.

An epitaxial substrate formed by epitaxially growing a Group III nitridelaminated body on a semiconductor substrate is generally used as anepitaxial substrate for a FET. Examples of such a semiconductorsubstrate as described above include: a Si substrate having specificresistance exceeding 10² Ω·cm for use to decrease substrate loss whichdeteriorates device performances, as disclosed in JP 2008-522447Laid-Open; and a Si substrate having specific resistance of 1.0 to 500Ω19 cm or so for use to decrease leak current to the Si substrate, asdisclosed in JP 2003-059948 Laid-Open.

It has been conventionally considered that use of a Si substrate havingrelatively high specific resistance is preferable, as described above.However, it has been known that, when layers having different specificresistance values are epitaxially grown on a Si substrate having apredetermined resistance, mismatch of lattice constants generally occursbetween the Si substrate and the epitaxially-grown layers, wherebywarp(s) is generated to alleviate strains. Such warpage of an epitaxialsubstrate as described above causes maladsorption and/or exposurefailure at the stage of a device process.

In order to solve the problems described above, JP 06-112120 Laid-opendiscloses a technique for decreasing the absolute value of warpage bydetermining in advance a warping direction in a semiconductor substrateand then adequately growing epitaxial layers on the substrate.

However, the technique disclosed in JP 06-112120 Laid-open simply aimsat decreasing the absolute value of warpage of an epitaxial substrateand determines in advance only warpage derived from a slicing process ofslicing a wafer from an ingot. Therefore, JP 06-112120 Laid-open cannotcontrol a final warp configuration of the epitaxial substrate in asufficient manner. JP 06-112120 Laid-open also has a problem that aproduction process thereof is complicated because it includes a processof determining a warping direction of the semiconductor substrate.

SUMMARY OF THE INVENTION Problems to be solved by the Invention

An object of the present invention is to solve the aforementionedproblems and provide an epitaxial substrate for an electronic device, inwhich substrate a lateral direction thereof is defined as a main currentconducting direction and a warp configuration thereof is adequatelycontrolled, and a method of producing the epitaxial substrate.

Means for Solving the Problems

In order to achieve the object described above, the present invention isprimarily structured as follows.

(1) An epitaxial substrate for an electronic device, including: a Sisingle crystal substrate; and a Group III nitride laminated body formedby epitaxially growing plural Group III nitride layers on the Si singlecrystal substrate, wherein a lateral direction of the epitaxialsubstrate is defined as a main current conducting direction, ischaracterized in that the Si single crystal substrate is a p-typesubstrate having a specific resistance value of not larger than 0.01Ω·cm.

The epitaxial substrate for an electronic device of (1) above, wherein asectional warp configuration of the epitaxial substrate satisfies arelationship formula below.

∥Bow|−SORI|≦2 μm

(3) The epitaxial substrate for an electronic device of (1) or (2)above, wherein a sectional warp configuration of the epitaxial substrateis monotonously bowed over the entire width of the epitaxial substrate.

(4) The epitaxial substrate for an electronic device of any of (1) to(3) above, wherein the Si single crystal substrate contains as animpurity element boron at a concentration of 10¹⁹/cm³ or higher.

(5) The epitaxial substrate for an electronic device of any of (1) to(4) above, further comprising a buffer as an insulating layer betweenthe Si single crystal substrate and the Group III nitride laminatedbody.

(6) The epitaxial substrate for an electronic device of (5) above,wherein the buffer includes a lamination constituted of a superlatticemultilayer structure.

(7) A method of producing an epitaxial substrate for an electronicdevice, in which a Group III nitride laminated body is formed byepitaxially growing plural Group III nitride layers on a Si singlecrystal substrate such that a lateral direction of the substrate isdefined as a main current conducting direction, comprising forming theSi single crystal substrate to be a p-type substrate having a specificresistance value of not larger than 0.01 Ω·cm by adding boron thereto ata relatively high concentration.

(8) The method of producing an epitaxial substrate for an electronicdevice of (7) above, wherein boron is added at a concentration of10¹⁹/cm³ or higher.

(9) The method of producing an epitaxial substrate for an electronicdevice of (7) or (8) above, further comprising: forming on the Si singlecrystal substrate a buffer as an insulating layer including a laminationconstituted of a superlattice multilayer structure, prior to formationof the Group III nitride laminated body; and forming the Group IIInitride laminated body having a HEMT structure on the buffer.

Effect of the Invention

According to the present invention, a warp configuration of an epitaxialsubstrate for an electronic device can be appropriately controlledwithout deteriorating performances of the device, by setting a specificresistance value of a Si single crystal substrate at a preferred valueor lower.

Specifically, according to the present invention, a warp configurationof an epitaxial substrate for an electronic device can be appropriatelycontrolled by setting a specific resistance value of a Si single crystalsubstrate at a preferred value or lower by adding boron to the Si singlecrystal substrate at a relatively high concentration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing a typical field effecttransistor (FET).

FIG. 2 is a schematic sectional view of an epitaxial substrate for anelectronic device according to the present invention.

FIG. 3 is a schematic view for explaining “BOW”.

FIG. 4 is a schematic view for explaining “SORI”.

FIGS. 5( a) to 5(d) are views of sectional warp configurations ofvarious types.

FIGS. 6( a) to 5(d) are views of sectional warp configurations ofvarious types.

FIGS. 7( a) to 7(d) are views each showing surface contour lines and asectional configuration of the epitaxial substrate for an electronicdevice, measured by using a configuration measuring device.

BEST MODE FOR IMPLEMENTING THE PRESENT INVENTION

An embodiment of an epitaxial substrate for an electronic device of thepresent invention will be described with reference to the drawingshereinafter. FIG. 2 schematically shows a sectional structure of anepitaxial substrate for an electronic device according to the presentinvention.

As shown in FIG. 2, an epitaxial substrate 1 for an electronic device ofthe present invention, including: a Si single crystal substrate 2; and aGroup III nitride laminated body 3 formed by epitaxially growing pluralGroup III nitride layers on the Si single crystal substrate 2, wherein alateral direction of the epitaxial substrate is defined as a maincurrent conducting direction, is characterized in that the Si singlecrystal substrate 2 is a p-type substrate having a specific resistancevalue of not larger than 0.01 Ω·cm. The present invention is based on adiscovery that, in an epitaxial substrate for an electronic device,having such a structure as described above, a warp configuration thereofcan be appropriately controlled without deteriorating performances ofthe device.

In the present invention, “a lateral direction is defined as a maincurrent conducting direction” means that electric current flows from thesoured electrode to the drain electrode, i.e. predominantly in thewidthwise direction of the laminated body, differing from, for example,a structure in which a semiconductor is interposed by a pair ofelectrodes such that electric current flows predominantly in thevertical direction, i.e. in the thickness direction of the laminatedbody.

A value of specific resistance of the Si single crystal substrate 2 isadjusted by adding a p-type impurity element to the Si single crystalsubstrate. Examples of the p-type impurity element include boron,aluminum, gallium, and the like. Among these examples, boron ispreferably used because boron can be added at a relatively highconcentration. In this regard, it is preferable to add boron at aconcentration of 10¹⁹/cm³ or higher in order to adjust a specificresistance value of the Si single crystal substrate 2 to not larger than0.01 Ω·cm. The dimension of the Si single crystal substrate 2 can beappropriately selected depending on applications. A face, for use, ofthe Si single crystal substrate is not particularly specified and anyface such as (111) face, (100) face, (110) face can be used. Use of(111) face is, however, preferable because then growth of (0001) face ofGroup III nitride is easily facilitated and surface-flatness of anepitaxial substrate improves. Further, it is acceptable to bond asubstrate made of another material on the back surface of the Si singlecrystal substrate and/or provide the Si single crystal substrate with aprotection film such as an oxide film, a nitride film, or the like.

A warp configuration of an epitaxial substrate for an electronic devicecan be optimized by adjusting a specific resistance value of the Sisingle crystal substrate 2 to not larger than 0.01 Ω·cm, as describedabove. In the present embodiment, appropriateness of a warpconfiguration is defined by the absolute value of a value obtained bysubtracting “SORI” as shown in FIG. 4 from the absolute value of “BOW”as shown in FIG. 3. A “BOW” value represents, as shown in FIG. 3, avalue obtained by: measuring the largest distance between a plane,passing through the center in the widthwise direction of a measurementsurface of an epitaxial substrate and in parallel with the best-fitreference plane, and the measurement surface in a non-adsorbed state ofthe epitaxial substrate; and imparting the distance value with a sign+/−depending on the vertical direction with respect to the center. On theother hand, a “SORI” value represents a distance in the verticaldirection between a plane, passing through the highest position of ameasurement surface of an epitaxial substrate and in parallel with thebest-fit reference plane, and a plane, passing through the lowestposition of the measurement surface and in parallel with the best-fitreference plane in a non-adsorbed state of the epitaxial substrate, asshown in FIG. 4.

A sectional warp configuration of the epitaxial substrate 1 for anelectronic device according to the present invention preferablysatisfies a relationship formula below.

∥Bow|−SORI|≦2 μm

It should be noted that peripheral portions of the epitaxial substrate,within 3 mm measured from edges of the substrate, are to be excluded inmeasurement of the BOW and SORI values because deformation in arelatively narrow range may be generated in the peripheral portion ofthe epitaxial substrate due to a SORI configuration of the Si singlecrystal substrate itself, a processed configuration of edges of thesubstrate, and the like.

FIGS. 5( a) to 5(d) are views of sectional warp configurations ofvarious types. FIGS. 6( a) to 6(d) are actually views of the samesectional warp configurations as those of FIGS. 5( a) to 5(d). Brokenlines in FIG. 5 and FIG. 6 are used to measure BOW values and SORIvalue. FIGS. 5( a) to 5(c) and FIGS. 6( a) to 6(c) each represent a casewhere a value of |Bow| and a value of SORI are equal to each other. FIG.5( d) and FIG. 6( d) each represent a case where a value of |Bow| and avalue of SORI are different from each other. A sectional warpconfiguration of the epitaxial substrate 1 for an electronic device ispreferably bowed monotonously in one direction, as show in FIG. 5( a)and FIG. 6( a). In contrast, in a case where a sectional warpconfiguration is bowed in two directions as shown in FIG. 5( d) and FIG.6( d), a value of |Bow| differs from a value of SORI, whereby the largerabsolute value of the difference between |Bow| and SORI results in themore unevenness in the two directions of the sectional warpconfiguration. If ∥Bow|−SORI| exceeds 2 μm, such an unevenly warpedconfiguration of the epitaxial substrate may deteriorate performances ofthe device and cause maladsorption and exposure failure at the stage ofa device process.

A sectional warp configuration of the epitaxial substrate for anelectronic device is preferably bowed monotonously over the entire widththereof, as show in FIG. 5( a) and FIG. 6( a), so that warp can beeasily corrected to suppress exposure failure of the device due tomaladsorption. The peripheral portions of the epitaxial substrate,within 3 mm measured from edges of the substrate, are to be excluded inconsideration of the sectional warp configuration of the epitaxialsubstrate, as described above.

It is preferable to further provide a buffer 4 as an insulating layerbetween the Si single crystal substrate 2 and the Group III nitridelaminated body 3. Provision of the buffer 4 as an insulating layerprevents electric current from flowing into the Si single crystalsubstrate 2, suppresses leak current in the vertical direction whichcould be facilitated by use of the Si substrate having relatively lowspecific resistance, and improves breakdown voltage of the substrate.

The buffer 4 preferably includes a lamination 4 a constituted of asuperlattice multilayer structure. The lamination 4 a may be formed asalternately laminated layers including at least a first layer 4 a ₁ madeof a B_(a1)Al_(b1)Ga_(c1)In_(d1)N material (0≦a₁≦1, 0≦b₁≦1, 0≦c₁≦1,0≦d₁≦a₁+b₁+c₁+d₁=1) and a second layer 4 a ₂ made of aB_(a2)Al_(b2)Ga_(a)In_(d2)N material (0≦a₂≦1, 0 b₂≦1, 0≦c₂≦1, 0≦d₂≦1,a₂+b₂+c₂+d₂=1) having a band gap different from that of the first layer.Differentiating a band gap of one layer from that of another layer, i.e.band discontinuities, contributes to increase in electric resistance inthe vertical direction. In this regard, the superlattice multilayerstructure preferably contains C (carbon) at a concentration of1×10¹⁸/cm³ or more because then generation of carriers due to banddiscontinuities is suppressed and breakdown voltage of the buffer can befurther improved. Although the upper limit of the C concentration is notparticularly limited, the upper limit thereof is preferably not higherthan 1×10²⁰/cm³ in terms of suppressing generation of pits in the GroupIII nitride laminated body 3. Formation of the conventional superlatticegenerally involves steep changes in interfaces thereof. The presentinvention, in addition to this, may further include cases where anotherlayer is inserted between interfaces, a composition of an interface iscontinuously varied, a composition of the superlattice multilayerstructure is varied, and the like, within a scope not marring thetechnical effect of the present invention.

In terms of improving breakdown voltage of the substrate, in particular,thickness of a layer having a relatively large band gap, of thelamination 4 a, is preferably equal to or larger than the thicknesscapable of suppressing tunneling current and equal to or smaller thanthe thickness which reliably avoids generation of cracks. For example, alayer having a relatively large band gap, of the lamination 4 a, ispreferably formed by using AlN having the largest band gap among theGroup III nitrides so as to have a thickness of 2 to 10 nm. Further,from a similar viewpoint, a layer having a relatively small band gap, ofthe lamination 4 a, preferably contains at least Al so that carbon canbe effectively incorporated at a sufficiently high concentration.

A layer having a relatively small band gap, of the lamination 4 a, ispreferably thicker than a layer having a relatively large band gap, ofthe lamination 4 a, and equal to or thinner than 40 nm in order toeffectively demonstrate a strain-alleviating effect of the superlatticemultilayer structure and suppress generation of cracks. Further, forsimilar reasons, difference in composition between a layer having arelatively small band gap and a layer having a relatively large bandgap, of the lamination 4 a, is necessary and difference in compositionof Al therebetween is preferably at least 50%, i.e. |b₁-b₂|≧0.5.

The number of laminated-layer pairs of the superlattice multilayerstructure is not particularly restricted. The larger number of suchpairs results in the better suppression of leak current in the verticaldirection and improvement of breakdown voltage.

Next, another embodiment regarding a method of producing an epitaxialsubstrate for an electronic device of the present invention will bedescribed with reference to the drawings. As shown in FIG. 2, theepitaxial substrate 1 for an electronic device of the present invention,in which a lateral direction of the epitaxial substrate 1 is defined asa main current conducting direction, is characteristically produced by:forming a Si single crystal substrate 2 to be a p-type substrate havinga specific resistance value of not larger than 0.01 Ω19 cm by addingboron thereto at a relatively high concentration; and epitaxiallygrowing plural Group III nitride layers on the Si single crystalsubstrate 2 to form a Group III nitride laminated body 3. Due to such astructure as described above, the epitaxial substrate for an electronicdevice of the present invention can adequately control a warpconfiguration thereof without deteriorating performances of the device.

It is preferable that boron is added at a concentration of 10¹⁹/cm³ ormore so as to evenly exist across the entire region of the substrate.Boron may be either added as an impurity to Si single crystal when theSi single crystal is produced by the CZ method, the FZ method or thelike, or introduced to the substrate by ion injection, thermaldiffusion, or the like. In the present embodiment, boron need not beadded across the entire region of the Si single crystal substrate at thepreferred B content described above and it suffices that boron is addedat the preferred content to at least a portion of the substrate. Forexample, the present invention includes an application where a substratecontaining boron at the aforementioned concentration or more is providedwith a Si film formed thereon of which boron content is lower than theaforementioned concentration and an application where portions having Bcontent lower than the aforementioned preferred content locally existwithin the Si substrate. Further, the present invention includes anapplication where a surface-modified layer such as a Si nitride film, acarbide film, an oxide film or the like is formed as an initial layer onthe substrate surface and an application where the substrate includes amaterial other than Si and Group III nitrides. Yet further, theepitaxial substrate for an electronic device of the present inventionmay contain impurities other than B, such as Al, Ga, In, P, Sb, As, H,C, Ge, N, O and the like. Impurities are preferably added in order toincrease hardness of the Si single crystal substrate.

It is preferable that a buffer as an insulating layer including alamination constituted of a superlattice multilayer structure is formedon the Si single crystal substrate prior to formation of the Group IIInitride laminated body and thereafter the Group III nitride laminatedbody having a HEMT structure is formed on the buffer. Each of thelamination of a superlattice multilayer structure and the Group IIInitride laminated body of a HEMT structure can be formed by a thin-filmlaminating method of various types, such as MOCVD, MBE, HYPE and thelike.

FIGS. 1 to 6 show typical examples of the embodiments and the presentinvention is not restricted to these illustrated examples of theembodiments.

EXAMPLES Example 1

A Si single crystal substrate of 3-inch diameter (plate thickness: 625μm, content of added boron: 2×10¹⁹/cm³: specific resistance: 0.005 Ω·cm,crystal face (111)) was heated in an atmosphere of hydrogen and nitrogenat 1050° C. Thereafter, an AlN layer having film thickness 200 nm and anAl_(0.25)Ga_(0.75)N layer having film thickness 50 nm were formed on theSi single crystal substrate by adjusting supply rates oftrimethylgallium (TMG), trimethylaluminum (TMA) and NH₃ by using theMOCVD method. Further, an insulating superlattice multilayer structureas a lamination of 80 pairs of alternately laminated an AlN film (filmthickness: 4 nm) and an Al_(0.15)Ga_(0.85)N film (film thickness: 25 nm)was formed on the Al_(0.25)Ga_(0.75)N layer by adjusting supply rates oftrimethylgallium (TMG), trimethylaluminum (TMA) and NH₃. The averageCarbon concentration of the superlattice multilayer structure was2×10¹⁸/cm³. A GaN layer of 1.5 μm thickness and an Al_(0.25)Ga_(0.75)Nlayer (film thickness 20 nm) functioning as lateral-direction currentconducting layers were formed on the superlattice multilayer structure,whereby an epitaxial substrate for an electronic device was prepared.

Example 2

An epitaxial substrate for an electronic device was prepared in the samemanner as in Example 1, except that the content of added boron was1×10¹⁹/cm³ and specific resistance of the Si single crystal substratewas 0.01 Ω·cm.

Comparative Example 1

An epitaxial substrate for an electronic device was prepared in the samemanner as in Example 1, except that the content of added boron was4×10¹⁸/cm³ and specific resistance of the Si single crystal substratewas 0.02 Ω·cm.

Comparative Example 2

An epitaxial substrate for an electronic device was prepared in the samemanner as in Example 1, except that the content of added boron was1.5×10¹⁶/cm³ and specific resistance of the Si single crystal substratewas 1 Ω·cm.

Comparative Example 3

An epitaxial substrate for an electronic device was prepared in the samemanner as in Example 1, except that the content of added boron was8×10¹⁴/cm³ and specific resistance of the Si single crystal substratewas 25 Ω·cm.

Comparative Example 4

An epitaxial substrate for an electronic device was prepared in the samemanner as in Example 1, except that the content of added boron was1×10¹³/cm³ and specific resistance of the Si single crystal substratewas 5000 Ω·cm.

Evaluation

A warp configuration of each of the epitaxial substrates for electronicdevices of Examples 1, 2 and Comparative Examples 1 to 4 was observed byusing a configuration measuring device (FT-900: manufactured by NIDECCorporation), so that the BOW value and the SORI value were obtained,respectively. FIGS. 7( a) to 7(d) show surface contour lines andsectional warp configurations of the epitaxial substrates of Example 1,Comp. Example 1, Comp. Example 2 and Comp. Example 4 obtained by theconfiguration measuring device, respectively. Table 1 show themeasurement results of the BOW values and the SORI values of theseepitaxial substrates.

Ten samples were prepared for each of the epitaxial substrates forelectronic devices of Examples 1, 2 and Comparative Examples 1 to 4 inorder to analyze variation in configuration of the Si single crystalsubstrate itself, and these samples were evaluated as described above.Table 2 show ranges from the minimum value to the maximum value of“specific resistance” and “∥Bow|−SORI|”, respectively.

TABLE 1 Specific resistance BOW SORI ||Bow| − SORI| (Ω/cm) (μm) (μm)(μm) Example 1 0.005 −17.89 17.94 0.05 Example 2 0.01 −18.90 19.00 0.10Comp. Example 1 0.02 −4.85 14.08 9.23 Comp. Example 2 1 −8.29 18.5610.27 Comp. Example 3 25 −8.29 23.56 15.27 Comp. Example 4 5000 30.6050.00 19.40

TABLE 2 Specific resistance ||Bow| − SORI| (Ω/cm) (μm) Example 1 0.005  0-0.5 Example 2 0.01 0-2 Comp. Example 1 0.02  5-15 Comp. Example 2 1 5-20 Comp. Example 3 25 10-30 Comp. Example 4 5000 15-40

It is understood from FIG. 7( a) that the sectional warp configurationof the epitaxial substrate for an electronic device of Example 1 of thepresent invention is monotonously bowed in one direction. It is alsounderstood from FIGS. 7( b) to 7(d) that the sectional warpconfigurations of the epitaxial substrates for electronic devices ofComp. Example 1, Comp. Example 2 and Comp. Example 4 are notmonotonously bowed in one direction. Further, it is understood fromTables 1 and 2 that Examples 1 and 2 according to the present inventioncan reduce a value itself of and variation in ∥Bow|−SORI|, as comparedwith Comp. Examples 1 to 4, by adjusting a specific resistance value ofthe Si single crystal substrate to not larger than 0.01 Ω·cm.

Thickness and dimensions of the epitaxial substrate for an electronicdevice of the present invention are not particularly restricted to thosein Examples described above and may be appropriately selected dependingapplications in use.

INDUSTRIAL APPLICABILITY

According to the present invention, a warp configuration of an epitaxialsubstrate for an electronic device can be appropriately controlledwithout deteriorating performances of the device, by setting a specificresistance value of a Si single crystal substrate at a preferred valueor lower.

EXPLANATION OF REFERENCE NUMERALS

-   1 Epitaxial substrate for electronic device-   2 Si single crystal substrate-   3 Group III nitride laminated body-   3 a Channel layer-   3 b Electron supply layer-   4 Buffer-   4 a Lamination constituted of superlattice multilayer structure-   4 b intermediate layer-   4 c Seed layer

1. An epitaxial substrate for an electronic device, including: a Sisingle crystal substrate; and a Group III nitride laminated body formedby epitaxially growing plural Group III nitride layers on the Si singlecrystal substrate, wherein a lateral direction of the epitaxialsubstrate is defined as a main current conducting direction, wherein:the Si single crystal substrate is a p-type substrate having a specificresistance value of not larger than 0.01 Ω·cm.
 2. The epitaxialsubstrate for an electronic device of claim 1, wherein a sectional warpconfiguration of the epitaxial substrate satisfies a relationshipformula below.∥Bow|−SORI|≦2 μm
 3. The epitaxial substrate for an electronic device ofclaim 1, wherein a sectional warp configuration of the epitaxialsubstrate is monotonously bowed over the entire width of the epitaxialsubstrate.
 4. The epitaxial substrate for an electronic device of claim1, wherein the Si single crystal substrate contains as an impurityelement boron at a concentration of 10¹⁹/cm³ or higher.
 5. The epitaxialsubstrate for an electronic device of claim 1, further comprising abuffer as an insulating layer between the Si single crystal substrateand the Group III nitride laminated body.
 6. The epitaxial substrate foran electronic device of claim 5, wherein the buffer includes alamination constituted of a superlattice multilayer structure.
 7. Amethod of producing an epitaxial substrate for an electronic device, inwhich a Group III nitride laminated body is formed by epitaxiallygrowing plural Group III nitride layers on a Si single crystal substratesuch that a lateral direction of the substrate is defined as a maincurrent conducting direction, comprising: forming the Si single crystalsubstrate to be a p-type substrate having a specific resistance value ofnot larger than 0.01 Ω·cm by adding boron thereto at a relatively highconcentration.
 8. The method of producing an epitaxial substrate for anelectronic device of claim 7, wherein boron is added at a concentrationof 10¹⁹/cm³ or higher.
 9. The method of producing an epitaxial substratefor an electronic device of claim 7, further comprising: forming on theSi single crystal substrate a buffer as an insulating layer including alamination constituted of a superlattice multilayer structure, prior toformation of the Group III nitride laminated body; and forming the GroupIII nitride laminated body having a HEMT structure on the buffer.